Express Safflower Tolerance at Hettinger, ND, 2024

A trial was conducted to evaluate herbicide tolerance in two varieties of safflower that had been selected for tolerance to the herbicide tribenuron-methyl (Express), a herbicide belonging to the sulfonylurea family which inhibits the ALS enzyme. Safflower was planted on May 16, 2024 into a field that had been previously treated in a combination of pendimethalin, sulfentrazone, and glyphosate to control weeds prior to planting. Safflower was planted using a no-till plot drill at a depth of 1.75 inches and a seeding rate of 20 LB/A. Safflower emerged on May 31. The herbicide tribenuron-methyl (Express) was applied at a rate of 0.0625 lbs active ingredient per acre (2 oz product per acre); a rate that is 4 times the labelled rate when used for weed control in sunflower. This high rate was used to verify the tolerance of safflower beyond what will typically be applied, but what may result in areas of a field where spray overlap occurs. Express was tank-mixed with the herbicide clethodim (Select Max) at 6 oz/a, and methylated seed oil (MSO) at 1%v/v. Treatments were applied to safflower plots on two application dates; June 26 and July 8. Safflower was evaluated for injury two weeks after each treatment timing. Safflower height was also measured two weeks after each herbicide application timing. At maturity, safflower was harvested using a small plot combine to record seed yield. No visible injury was observed during either visual injury evaluation. While there were differences in safflower height, this was mostly due to differences between varieties. Within variety two, safflower height was slightly less when comparing the earlier treatment with the later treatment. However, these treatments were not statistically shorter in height when compared with the untreated control. Safflower yield was numerically lowest in the untreated control, but in most cases, these differences were not significant. Safflower test weight was greater in Variety One compared with Variety Two. Also seed test weight was less in the untreated safflower control compared with the later application timing. Seed yield of safflower was lower than what is typically expected in southwest North Dakota due to the hot and dry weather that occurred during the summer of 2024. The months of July and August were exceptionally hot and dry. From this trial, we saw no adverse effect of applying tribenuron-methyl to these two safflower varieties.